The present invention concerns rechargeable batteries, and more particularly batteries whose electrodes involve conjugated electroconductive organic polymers, as well as a method for conditioning such polymer electrodes.
It is well known that all batteries are formed of a plurality of cells each including at least two electrodes, a negative electrode (defined as the anode) and a positive electrode (defined as the cathode), both being immersed in an ionic conducting liquid (the electrolyte). During discharge of the cell, the electrons leave the anode, flow through an external circuit connected to the electrodes where they do work, and return to the cell via the cathode whose positive charge is thus progressively neutralized. This process continues until an equilibrium is reached, i.e. until when the electron donating substances (at the anode) and electron acceptor substances (at the cathode) are consumed, or when an opposing potential arises at the electrodes due to the presence in the electrolyte of the electrooxidized and corresponding electroreduced products which are formed in the reaction.
For instance, in the classical nickel/cadmium battery in which the cadmium is the anode and nickel oxide is the cathode, the cadmium spontaneously dissolves into an electrolyte solution to form cadmium ions Cd.sup.2 +, giving up electrons to the external circuit while the nickel oxide (Ni.sup.III) is reduced to Ni.sup.II by the incoming electrons. In the electrolyte (aqueous alkali hydroxide solution), the negative charges are carried toward the cadmium electrode (then called "anode") by the hydroxy anions, whereas the positive Cd.sup.2 + cations move toward the other electrode (then called "cathode").
In recharging, the reverse operations take place: the Cd.sup.2 ++ cations travel the opposite way in the electrolyte to be reduced back to metallic cadmium at the negative electrode (which is defined then as a cathode), while the negative OH- anions go back to the nickel electrode (the anode in this case) where they reoxidize Ni.sup.II to Ni.sup.III. Hence the operation of the Ni/Cd battery involves chemical consumption of electroactive substances in discharge, and upon recharging the original substances are re-formed from the thus chemically modified species, e.g., the cadmium metal is plated out of the cadmium ion solution. Now, cadmium is very toxic and undesirable; however, when replaced by zinc, the reversibility of the above operations is sometimes awkward (dendrite formation) and the number of charge and discharge cycles is rather limited; hence, batteries using polymer electrodes not subjected to periodical consumption and re-formation of electroactive substances are attractive because of their inherent reversibility and prolonged lifetime.
For instance, in cells involving carbon electrode substrates, polyacetylene (or other conjugated electroconductive polymers) electrode materials and an alkali salt, e.g., LiClO.sub.4 in a non-aqueous solvent as the electrolyte, the following phenomena occur when charging: the generator pumps off electrons from the positive electrode and drives them through the external circuit to the negative electrode where they "equilibrate" with the positive Li+ cations from the electrolyte (the electrode is doped). Simultaneously, the positive "holes" in the positive electrode are neutralized, and thus the electrode is doped with ClO.sub.4 - anions. During discharge the reverse effect takes place.
Among the organic electroconductive polymers, polypyrrole is a favored one because of its long recycling life and easy electrochemical or chemical preparation. So many battery systems involving polypyrrole (pPy) have been reported. Many are hybrid systems in which the positive electrode comprises polypyrrole and the negative electrode is made of an electropositive consumable metal such as alkali metals, metals of Groups II and III of the Periodical Table or alloys thereof (see EP-A-199 175; ALLIED CORP). For illustration, some prior art references are briefly reviewed below.
For instance, Japanese Patent Laid-Open No. 60-225376 (1985), TOYOTA MOTOR CORP., discloses a positive electrode made of carbon fibers coated with pPy or polythiophene and, optionally, other conductive materials such as gold, copper, silver, In.sub.2 O.sub.3, SnO.sub.2, and the like. For making a cell, an Al counterelectrode is used in an LiClO.sub.4 /acetonitrile solution. On charge, the lithium deposits on the negative electrode, while the positive pPy electrode is doped with ClO.sub.4 - ions. A cell with open voltage of 2.5-3 V is thus obtained (the lithium anode is about 2-2.5 V below the Ag/AgCl reference couple).
Japanese Patent Laid-Open No. 62-170150, TOYOTA MOTOR CORP., discloses a battery with stacked electrode couples. The electrodes are similar to that of the previous reference, the use of some further electroactive polymers being listed, e.g., polyaniline, polythiophene (polythienylene) and the like. Listed electrolytes include LiClO.sub.4, R.sub.4 NClO.sub.4, R.sub.4 NPF.sub.6, R.sub.4 NBF.sub.4 (R being alkyl) in solvents like acetonitrile, propylene carbonate, benzonitrile, nitromethane, sulfolane and mixtures thereof.
The positive electrodes are manufactured from pieces of knitted carbon fibers which are dipped in 1-2 molar pyrrole/acetonitrile solution containing LiBF.sub.4 (2 molar) and electrolyzed against an Al counterelectrode at 7 mA/cm.sup.2 for an hour. This provides a polypyrrole coat of a few hundred of .mu.m on the graphite knit.
Japanese Patent Laid-Open No. 60-127663, TOYOTA CENTRAL RESEARCH INST., discloses a battery in which the cathode comprises a coat (1-1000 .mu.m thick) of polymer blend or of copolymer of pyrrole and thiophene or alkyl derivatives thereof deposited on current collectors which can be of platinum, gold, nickel, steel, graphite, carbon and the like. The technique for depositing the polymers on the collectors is similar to that disclosed in the previous references using an aluminum negative electrode and lithium salts in polar organic solvents as the electrolyte. Films of electroactive polymers in the range of 100 .mu.m thickness are deposited under at a current density of 7 mA/cm.sup.2. Current densities around 12 mA/cm.sup.2 or 1 mA/cm.sup.2 are reported to be too high or too low, respectively.
Japanese Patent Laid-Open No. 61-163562 (1986), BRIDGESTONE CO., proposes a battery with an electroconductive polymer cathode and an anode of carbon material which dopes upon charging with cations. As such carbon materials cellulose or phenolic resins with conductivities above 10.sup.-4 S/cm are convenient.
The cathode material disclosed in this reference includes polyaniline, polyphenylene, polyfuran, polypyrrole and others. Electrolytes to be used here include alkali metal salts of ClO.sub.4 -, PF.sub.6 -, AsF.sub.6 -, BF.sub.4 -, CNS-, SO.sub.4.sup.2 - and the like in solvents such as propylene and ethylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran (THF), .gamma.-butyrolactone, dioxane, MeCl.sub.2, trialkyl-phosphates and -phosphites, DMF, DMSO, dichloroethane, chlorobenzene and the like. In Example of this application, 2.7 mg of carbon textile material was doped for 12.5 hours at 50 .mu.A in LiClO.sub.4 /propylene carbonate solution against a polyaniline electrode. The charging capacity was 98 Ahr per one kg of polyaniline, 233 Ahr per one kg of carbon and 70 Ahr per kg of both electrode materials. The open circuit voltage was 3.8 V, and the cell voltage was 3.2 V under 50 .mu.A discharge current. The final cell voltage was 2 V and after 50 recycles, no metallic Li was deposited on the anode. The Coulomb efficiency is indicated to be 86%.
Japanese Patent Laid-Open No. 62-176046 (1987), MITSUBISHI CHEM. & SANYO ELECTRIC, discloses secondary batteries in which either the anode or the cathode or both are made of electroconductive polymers, inter alia, polypyrrole and polythiophene, the polymers being deposited into porous substrates to avoid them from becoming disintegrated under use conditions. The reference also discloses in its introductory part that batteries in which both electrodes are made of electroconductive polymers are already proposed by Japanese Patent Publication No. 60-216471 (1985).
Porous substrates indicated in Japanese Patent Laid-Open No. 62-176046 include expanded metals and alloys such as Ni, Ni-Cr, Ni-Cu, N-Fe-Co, Fe-Cr, Cu, Fe, Pb, Cd, Au, Ag and others. Embodiments relate to cells in which the anode is of lithium and the cathode of electropolymerized pyrrole. Capacities of 28 mAhr/cm.sup.2 are reported after charging under 7 mA/cm.sup.2 in solutions of LiClO.sub.4 in propylene carbonate. No workable details are, however, given for cells in which both electrodes are of polypyrrole.
Japanese Patent Laid-Open No. 61-128478, TOYOTA MOTOR CORP., reports a method for the production of negative electrodes of electrically conductive polymers, namely polypyrrole. This reference also reports in its introductory part that it is already known by Denki Kagaku 52 (1984), pp. 80-81, issued by the Japan Electrochemistry Association that polythienylene is previously doped with quaternary ammonium ions to form an "n" type electroactive material. However, the doping was reported not extensive. The materials disclosed in Japanese Patent Laid-Open No. 61-128478 include alkali metal trifluoromethane-sulfonate, BF.sub.4 - and PF.sub.6 - solutions in acetonitrile, DMF, propylene carbonate, THF, hexamethylphosphoramide and the like as electrolytes. The electrode current collector substrates can be made of Pt or carbon fibers. In Example of this reference, a graphite fiber sheet was coated electrolytically with pPy in a 0.2M solution of pyrrole in acetonitrile (0.2M Bu.sub.4 NCF.sub.3 SO.sub.3 as supporting electrolyte), a Pt sheet being used as the negative counterelectrode. Hence, the obtained pPy electrode was a positively charged, anion-doped material. This polypyrrole electrode was then converted to a negatively charged material by immersing into a 0.2M Et.sub.4 NClO.sub.4 solution in DMSO and charging against a Pt counterelectrode under 1-10 mA for 1 hour or more. The doping ratio indicated reached 10 mol % calculated on pPy and a terminal discharge voltage of 1.5 V versus the Ag/AgCl reference couple was reported.
When the present inventors attempted to repeat the foregoing experiments, they noted that the reported results were not attained; for instance, the amount of charging and doping was small, and the anode thus conditioned could not be used in the manufacturing of commercially workable batteries.